In the midst of the vast, relatively unpopulated ocean, coral reefs are hotspots of biological diversity. Thousands of species are hosted within their colorful, protective borders. They support many of the fisheries that coastal communities rely upon and sustain local economies by providing billions of dollars of income from tourism.

The vibrant colors that attract both fish and humans are the result of a symbiotic relationship between invertebrate coral polyps and colorful algae. Nutrients and protection are provided by corals to algae. In return, the algae provide the corals with sugars produced via photosynthesis.

In 1979, incidents in which coral hosts cast out their resident algae began to be reported on an unprecedented scale. This occurrence, known as coral bleaching, leaves the coral a bleached white color because of the ejection of the colorful algae. Bleaching events are not necessarily lethal to corals. However, if bleaching occurs over a prolonged period of time, the coral host will eventually die.

Explanation:

The subject-verb agreement is a grammar rule according to which the subject and verb must agree in number. This means that, if the subject is singular, the verb must be singular as well. If the subject is plural, the verb should be too.

For example, reefs is a plural noun. It needs to be followed by a plural verb, which means that the verb is is incorrect. Here, the plural form are should be used. The principle is similar in the rest of the cases, which I've written in bold letters and underlined. The number of the verb needs to be changed.

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